The subject matter disclosed herein generally relates to electrochemical sensors and, more particularly, to state of health monitoring and restoration of electrochemical sensors.
Electrochemical gas sensors are gas detectors that measure a concentration of a target gas by oxidizing or reducing the target gas at an electrode and measuring the current that is generated. Electrochemical gas sensors are normally designed to operate in a diffusion limited mode. This is normally achieved by using a capillary or membrane, which limits gas access in a well-defined and repeatable way. The sensor is designed such that the capillary or membrane provides the limiting factor. For example, the gas sensing electrode is designed to have sufficient activity reserve that the actual activity of the electrode can generally be ignored (since it is much greater than required to consume the available gas). Under certain conditions, however, sensors can deviate from the ideal diffusion limited behavior: for example, if the catalytic activity of the electrode degrades significantly, or if blocking or flooding occurs, then the sensor response may deviate from the predetermined relationship between concentrations of species to be detected and an analog output.
In addition to such faults which can result in a change in the level of gas response, certain faults can result in a reduction in the speed of response, even though the steady state response remains unchanged: for example, if the membrane electrode assembly becomes partially flooded or poisoned this may result in a slow response, and in some cases a reduction in steady state response. The same is true if the catalytic activity of the electrode is reduced, which can be a particular problem for sensors with low activity reserve.
An example of a gas that may be monitored, and related sensors which may be subject to faults, is low global warming potential (LGWP) refrigerants that may be used in HVAC systems. Various LGWP refrigerants are mildly flammable, and thus leak detection is desired. Electrochemical sensors have been successfully applied for toxic gas sensing. Passive fault mitigation methods such as adsorption by activated carbon have been used successfully to protect CO sensors from poisoning, i.e., electrode poisoning or saturation or coverage by an undesirable chemical. Such a method can be ineffective when the targeted species adsorbs on the adsorbent, resulting in substantial interference of the sensing process or completely defeat the detectability of the sensor. The electrochemical sensor failure mechanisms resulting from catalyst deactivation due to ambient volatile species can be extremely difficult to detect by the sensors, particularly when the sensors are deployed for leak detection expected to occur rarely. This is because during catalyst deactivation a sensor can generate a similar response as if a leak is detected and low cost electrochemical sensors may lack the ability to determine the identification of the species detected. However, it would be necessary to deploy a fail-safe operation of the sensors for safety monitoring, e.g., for leak detection of flammable refrigerants. Therefore, it is may be desirable to enable an electrochemical sensor to examine the state of health of the sensor pertaining to catalyst activity and restore the sensor's activity when the state of health is compromised due to exposure to active species.